The bellows was not cardboard. It was formed by sandwiching a rather intricate skeleton of heavy paper or card stock between an inner layer of opaque black fabric (impregnated with rubber or something similar) and an outer layer of very thin leather, the whole ensemble held together with a flexible adhesive.

The skeleton (think of a piece of manila file folder with a lot of slits cut in it, those slits representing the folds of the bellows) may still be intact, which can save you a good deal of difficulty.

You can make a replacement if you can find the materials (which may take some doing, but is not impossible). The late Ed Romney wrote a useful book on bellows-making, probably still available from the Romney website.

There is a good deal of information on this site, too.

There also are several outfits that will make a replacement bellows to your specification. The "Series B" bellows is not large and, if I may judge from one I owned while I was in college, the dimensions are not super-critical. A replacement shouldn't break the bank.